1. Field of the Invention
The invention relates to a high-temperature protection layer of an alloy with a base of chromium, aluminum and cobalt adapted particularly for protecting parts of gas turbines subjected to high temperatures.
2. Description of the Prior Art
Such high-temperature protection layers find application in protecting the base material of parts or structural elements of heat-resistant steels and/or alloys which are used at temperatures above 600.degree. C. These high-temperature protection layers are to retard the effect of high-temperature corrosion particularly of sulfur, oil ash, oxygen, earth alkalies and vanadium. The high-temperature protection layers are applied directly to the base material of the part. High-temperature protection layers are of particular importance in structural elements or parts of gas turbines. They are applied particularly to rotor and guide blades, as well as to segments of gas turbines subject to localized heat. In the manufacture of these parts, an austenitic material based on nickel, cobalt or iron is preferably used. In the manufacture of gas turbine parts, nickel super alloys particularly are used as the base material. The high-temperature protection layers to be applied consist preferably of chromium-containing alloys.
A high-temperature protection layer with a base material which contains cobalt, chromium and aluminum is known. This high-temerature layer is preferably applied to parts which are subjected to temperature effects substantially above 900.degree. C. The structure of such a high-temperature protection layers has a matrix consisting of cobalt, chromium and aluminum, into which a cobalt-aluminum-containing phase is incorporated. This high-temperature protection layer has the property, under operating conditions in which the high-temperature protection layer is thermally stressed of forming a passive cover layer of aluminum oxide on its surface. If this high-temperature protection layer is subjected to the action of air and to a temperature of 950.degree. for a time, corrosion occurs with the gradual removal of the passive cover layer. In the course of time upon the continuation of the high temperature-air conditions, the corrosion continues and also attacks the matrix. As a consequence, the cobalt-aluminum-containing phase which determines the mechanical strength of the high-temperature protection layer is removed in time, and the entire high-temperature protection layer destroyed.